1. The Field of the Invention
The present invention relates to x-ray generating devices and their method of manufacture. More particularly, the present invention relates to an x-ray tube having an evacuated housing assembly that provides enhanced thermal stability and improved x-ray shielding characteristics. The invention also relates to methods of manufacturing the improved housing assembly.
2. The Prior State of the Art
X-ray generating devices are extremely valuable tools for use in a variety of medical and industrial applications. For example, such equipment is commonly used in areas such as medical diagnostic and therapeutic radiology.
Regardless of the particular application involved, the basic operation of x-ray devices is similar. In general, an x-ray generating device is formed with a vacuum housing that encloses an anode assembly and a cathode assembly. The cathode assembly includes an electron emitting filament that is capable of emitting electrons. The anode assembly provides an anode target that is axially spaced apart from the cathode and oriented so as to receive electrons emitted by the cathode. In operation, electrons emitted by the cathode filament are accelerated towards a focal spot on the anode target by placing a high voltage potential between the cathode and the anode target. These accelerating electrons impinge on the focal spot area of the anode target. The anode target is constructed of a high refractory metal so that when the electrons strike, at least a portion of the resultant kinetic energy generates x-radiation, or x-rays. The x-rays then pass through a window that is formed within a wall of the vacuum enclosure, and are collimated towards a target area, such as a patient. As is well known, the x-rays that pass through the target area can be detected and analyzed so as to be used in any one of a number of applications, such as a medical diagnostic examination.
In general, only a very small portion—approximately one percent in some cases—of an x-ray tube's input energy results in the production of x-rays. In fact, the majority of the input energy resulting from the high speed electron collisions at the target surface is converted into heat of extremely high temperatures. In addition, a percentage of the electrons that strike the anode will rebound from the target surface and strike other areas within the x-ray tube assembly. The collisions of these secondary electrons (sometimes referred to as “back-scattered electrons) also create heat and/or result in the production of errant x-rays. This excess heat is absorbed by the anode assembly and is conducted to other portions of the anode assembly, and to the other components that are disposed within the vacuum housing. Over time, this heat can damage the anode, the anode assembly, and/or other tube components, and can reduce the operating life of the x-ray tube and/or the performance and operating efficiency of the tube.
Several approaches have been used to help alleviate problems arising from the presence of the high operating temperatures in the x-ray tube. For example, in some x-ray devices the x-ray target, or focal track, is positioned on an annular portion of a rotatable anode disk. The anode disk (also referred to as the rotary target or the rotary anode) is then mounted on a supporting shaft and rotor assembly, that can then be rotated by some type of motor. During operation of the x-ray tube, the anode disk is rotated at high speeds, which causes the focal track to continuously rotate into and out of the path of the electron beam. In this way, the electron beam is in contact with any given point along the focal track for only short periods of time. This allows the remaining portion of the track to cool during the time that it takes to rotate back into the path of the electron beam, thereby reducing the amount of heat absorbed by the anode.
While the rotating nature of the anode reduces the amount of heat present at the focal spot on the focal track, a large amount of heat is still present within the anode, the anode drive assembly, and other components within the evacuated housing. This heat must be continuously removed to prevent damage to the tube (and any other adjacent electrical components) and the x-ray tube's efficiency and overall service life.
One approach has been to place the housing that forms the evacuated envelope within a second outer metal housing, which is sometimes referred to as a “can.” This outer housing must serve several functions. First, it must act as a radiation shield to prevent radiation leakage, such as that which results from back-scattered electrons previously discussed. To do so, the can must include a radiation shield, which must be constructed from some type of dense, x-ray absorbing metal, such as lead. Second, the outer housing serves as a container for a cooling medium, such as a dielectric oil, which can be continuously circulated by a pump over the outer surface of the inner evacuated housing. As heat is emitted from the x-ray tube components (anode, anode drive assembly, etc.), it is radiated to the outer surface of the evacuated housing, and then at least partially absorbed by the coolant fluid. The heated fluid is then passed to some form of heat exchange device, such as a radiative surface, and then cooled. The fluid is then re-circulated by the pump back through the outer housing and the process repeated.
The dielectric oil (or similar fluid) may also provide additional functions. For example, the oil serves as an electrical insulator between the high voltage potential that exists at the anode and cathode assemblies and the inner evacuated housing, and the outer housing, which is typically comprised of a conductive metal material that is at a different potential, typically ground.
While useful as a heat removal medium and/or as an electrical insulator, the use of oil and similar liquid coolants/dielectrics can be problematic in several respects. For example, use of a fluid adds complexity to the construction and operation of the x-ray generating device. Use of fluid requires that there be a second outer housing or can structure to retain the fluid. This outer housing must be constructed of a material that is capable of blocking x-rays, and it must be large enough to be completely disposed about the inner evacuated housing to retain the coolant fluid. This increases the cost and manufacturing complexity of the overall device. Also, the outer housing requires a large amount of physical space, resulting in the need for an overall larger x-ray generating device. Similarly, the space required for the outer housing reduces the amount of space that can be utilized by the inner evacuated housing, which in turn limits the amount of space that can be used by other components within the x-ray tube. For example, the size of the rotating anode is limited; a larger diameter anode is desirable because it is better able to dissipate heat as it rotates.
Moreover, construction of the outer housing adds expense and manufacturing complexity to the overall device in other respects. If the liquid is used as a coolant, the device may also be equipped with a pump and a radiator or the like, that in turn must be interconnected within a closed circulation system via a system of tubes and fluid conduits. Also, since the fluid expands when it is heated, the closed system must provide a facility to expand, such as a diaphragm or similar structure. Again, these additional components add complexity and expense to the x-ray device's construction. Moreover, the tube is more subject to fluid leakage and related catastrophic failures attributable to the fluid system.
The presence of a liquid coolant/dielectric is also detrimental because it does not function as an efficient noise insulator. In fact, the presence of a liquid may tend to increase the mechanical vibration and resultant noise that is emitted by the operating x-ray tube. This noise can be distressing to the patient and/or the operator. The presence of liquid also limits the ability to utilize other, more efficient materials for dampening the noises emitted by the x-ray tube due to space restrictions and the need for effective electrical insulation.
Finally, use of a dielectric oil type of material is also undesirable from an environmental standpoint. In particular, the oil can be toxic, and must be disposed of properly.
Some prior art x-ray tubes have eliminated the use of an outer housing and fluid as a coolant/dielectric medium, and instead use only a single evacuated housing to enclose the x-ray tube components. Use of a single evacuated housing is advantageous in several respects. For example, eliminating the outer housing reduces the number of components required for the device. This results in a x-ray generating device that is more compact, that is lower in overall cost, that is less complex and easier to manufacture, and that is more reliable. In particular, elimination of the fluid coolant/dielectric reduces complexity and reduces the potential failure points noted above.
However, notwithstanding the recognized advantages of an x-ray generating device having a single evacuated housing, there are a number of problems that have limited its practicability. For example, to prevent excessive radiation from leaking from the x-ray tube, especially in high voltage applications, the housing must be equipped with a layer of x-ray absorbing material, such as a lead liner. However, this adds cost and manufacturing complexity to the device, because the lead shielding must be attached to the housing walls. Similarly, attachment of such a shield creates additional potential failure points that can reduce the reliability of the tube. For example, the shield layer should possess a thermal expansion rate that matches closely that of the underlying substrate material of the housing, or the materials can easily separate in the presence of the extreme temperature fluctuations of the operating x-ray tube.
Moreover, especially in high voltage applications, the use of some x-ray shields or liners substantially adds to the thickness of the housing walls, which takes up physical space and results in an overall larger x-ray tube. Again, this limits the amount of space that could otherwise be used by other x-ray tube components, such as a larger diameter anode.
Moreover, use of lead, or similar materials such as beryllium, as a liner material may again be undesirable due to environmental and health concerns relating to the toxicity of the substance. However, other suitable materials can be extremely expensive, can be difficult to manipulate during manufacturing, and/or may not possess satisfactory thermal characteristics for use in an x-ray tube.
To summarize, prior art x-ray generating devices typically rely upon the use of a second outer housing to provide a variety of functions, including cooling of the x-ray tube with a coolant, and preventing excessive radiation emissions. This outer housing adds cost and complexity to the x-ray generating device, and can reduce its long term reliability. While use of a single integral housing would thus be preferable, that approach also has drawbacks. In particular, the approach requires the use of a layer of x-ray shielding material, such as lead, on the housing walls to prevent unwanted radiation emissions. This adds cost and manufacturing complexity to the device, increases its overall size, and may not be desirable from an environmental and safety standpoint.
Thus, what is needed in the art, is a radiographic device, and a method for manufacturing the device, that does not require the use of an outer housing for containing oils or similar fluids for the removal of heat and/or for providing electrical insulation. Moreover, it would be an advancement in the art to provide a radiation generating device that uses a single evacuated housing that is capable of maintaining safe levels of radiation containment without using lead shields and the like.